In Patent Literature 1, there is a description of supplying a high-frequency low voltage to a compressor during shutdown at the time of heating. In Patent Literature 2, there is a description of supplying a single-phase AC voltage having a frequency higher than that at the time of a normal operation to a compressor, when the ambient temperature of an air conditioning unit becomes low.
In Patent Literatures 1 and 2, by applying a high-frequency AC voltage to a compressor according to decrease in the outside air temperature, the compressor is heated or kept warm, thereby facilitating a lubrication action inside the compressor.